Analysis of the ATLAS RPCs ROD timing performance with an embedded microprocessor

The readout data of the ATLAS' RPC Muon spectrometer are collected by the front-end electronics and transferred via optical fibres to the Read Out Driver (ROD) boards in the counting room. Each ROD arranges all the data fragments of one sector of the spectrometer in a unique event. This is made by the Event Builder logic, a cluster of Finite State Machines that parses the fragments, checks their syntax and builds an event containing all the sector data. In this paper we describe the Builder Monitor, developed to analyze the Event Builder timing performance. It is designed around a 32-bit softcore microprocessor embedded in the same FPGA hosting the Builder logic. This approach makes it possible to track the algorithm execution in the field. The Monitor performs real time and statistical analysis of the state machine dynamics. The microprocessor is interfaced with custom peripherals which read out the state registers, fill histograms and transfer them via DMA to the processor memory. The Builder Monitor also measures the elapsed time for each event, its length and keeps track of status and error words. We describe the hardware-software co-design of the Builder Monitor and the role played by the custom peripherals.